[Webkit-unassigned] [Bug 219808] New: Introduce RenderLayerScrollableArea

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=219808

            Bug ID: 219808
           Summary: Introduce RenderLayerScrollableArea
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: zimmermann at kde.org
                CC: bfulgham at webkit.org, simon.fraser at apple.com,
                    zalan at apple.com

All overflow/scroll related functionality should be moved out of RenderLayer, to cleanup and simplify the layer code.
This allows to optimize for the case where a layer is not scrollable: we no longer have to keep track of many scrolling
related conditions/variables and save a good amount of memory per layer.

Plan:
1) Introduce RenderLayerScrollableArea inheriting from ScrollableArea, with a back-reference to RenderLayer -- that mimics the design of RenderLayerFilters.
2) RenderLayer no longer needs to inherit from ScrollableArea, instead it stores a RenderLayerScrollableArea object.
3) Move all overflow/scroll/marquee/resize/... handling from RenderLayer to RenderLayerScrollableArea
4) Only create RenderLayerScrollableArea objects if necessary!
   This requires heavy testing of the regular and composited code paths, which I had to perform in the past weeks.

--

This ticket adds a RenderLayerScrollableArea stub implementation and adds it to all build systems.

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